Erbium-doped optical fiber amplifiers are known, e.g. from Document FR 2 638 854, as are associated undersea transmission systems. Such amplifiers enable high gains to be obtained merely by passing the signal along an optical fiber, and this applies to the spectrum range 1.5 .mu.m to 1.6 .mu.m. However, it is known (see J. F. Marcerou et al., Proceedings SPIE 1373, pp. 168-186, 1991), that when it is desired to make full use of an erbium-doped optical fiber amplifier, it is limited by the optical noise it generates, known as "amplified spontaneous emission" (ASE). Amplified spontaneous emission is then responsible in part for saturating the laser medium, which happens to the detriment of amplifying the signal(s) present in the optical fiber. Indeed, that is why ASE is used for making excellent wideband superfluorescent sources.
Several solutions have already been proposed for solving this problem. To reduce the noise factor, it is possible to use short fibers, but that penalizes gain. It is also possible to use fibers that are not optimized, but that requires pumping powers that are too high.
Proposals have also been made to limit the effect of amplified spontaneous emission in amplifiers by inserting an optical filter in the length of erbium-doped fiber. The optical filter blocks a spectrum component of the noise in both directions. However, that method suffers from the drawback of limiting the passband of the amplifier and is risky when amplifiers are used in cascade due to the fact that the frequencies differ a little from one filter to another.
In order to mitigate these drawbacks, the present invention proposes using an isolator (one or more) and inserting it within the amplifying optical fiber. This serves to reduce the deleterious effects of optical noise.